This invention relates to a thread feed mechanism in a sewing machine which is adapted to feed a predetermined length of thread necessary for forming one seam at one time from a supply source of thread to a thread take-up lever by a paying-out device which operates based on set signals and more particularly, to computing of the set signals.
In the sewing machine thread feed mechanism of the above type, the length l of thread necessary for forming one seam can be sought by the equation l=K.sub.1 P+K.sub.2 D+Z, and seam pitch is given by the equation P=.sqroot.x.sup.2 +y.sup.2, wherein X is workpiece feed pitch, Y is amplitude of needle bar, D is sewn workpiece thickness, Z is thread amount necessary for entwining and K is coefficient.
One example of thread paying-out devices which operate based on signals computed by the above-mentioned equation comprises two holding means for releasably pinching an indefinite length of thread and a thread paying-out member disposed in the thread path extending between the two holding means for operating in synchronization with a thread take-up lever whereby when the lever is not straining the thread, one of the holding means adjacent to a supply source of thread opens to release the thread from the nip whereas the other holding means adjacent to the lever closes to increase the length of the thread path extending between the two holding means so as to pay the thread out of the supply source, in response to the paying-out of the thread by a preset amount, the holding means adjacent to the supply source closes to prevent the thread from being further paid out and the holding means adjacent to the lever opens and the paid out thread is drawn out of the zone between the two holding means as the lever rises again. In another example of the thread paying-out devices of the type referred to above, a disc having a length of thread wound about the periphery of the disc is provided or a pair of rollers are provided in opposing and spaced relationship to releasably pinch the thread therebetween and at least one of the rollers is intermittently rotated by a stepping motor each time one seam is formed. In a further example of the thread paying-out devices of the type referred to hereinabove, a pair of spaced thread guides are provided in the thread path, a reciprocal paying-out member is provided to traverse the thread path and the movement amount of the reciprocal paying-out member is related to the movement amount of drive means such as a pulse motor or linear motor. However, the prior art thread paying-out devices referred to above have the disadvantage that since the balance in tension between upper and lower threads can not be varied as desired, the thread paying-out devices are not applicable to some sewing operations in which a single workpiece is sewn with gathers and two workpieces having different lengths are sewn together with the longer workpiece gathered.
In sewing machines, conventionally, the workpiece feed amount has been mechanically adjusted to square the feed amount in the forward direction with that in the reverse direction. However, differences between absolute feed amounts in the forward and reverse directions due to variation in tolerances of parts or disaccordance between an actual workpiece feed amount and a predetermined workpiece feed amount corresponding to the position of a feed adjusting device or a workpiece feed amount indicated by an adjusting dial or the like interlocked with the feed adjusting device could not be adjusted.
Therefore, in the workpiece feed amount adjusting system described just above, when a signal representing the detected position of the workpiece feed amount adjusting device and a control signal of a pulse motor which is adapted to displace the adjusting device are employed as workpiece feed amount signals for computing a thread paying-out amount, excess and deficiency in thread feed amount occur resulting in a low quality sewn product.